1. Field of the Invention
The present invention relates to an improvement in a color liquid crystal display apparatus, and particularly concerns a color liquid crystal display apparatus having twisted nematic liquid crystal combined with a number of color filter elements.
The present invention is usable for color picture display apparatus for a television receiver set, a monitor in a videotape recorder, a view finder of a television camera or a display apparatus of a computer terminal machine.
2. Description of the Prior Art
A color fluid crystal display apparatus has such a characteristics as having small thickness, operable with low voltage power source and consumes very small power, and accordingly its market is large. Furthermore, demand is rapidly growing these years as a flat type display apparatus. Developing from the conventional monocolor type liquid crystal display apparatus, color liquid crystal display apparatuses which uses a number of color filter elements are about to be introduced into actual use.
As operation modes of such color liquid crystal display apparatus for use in color displaying, dynamic scatterinc mode (hereinafter is referred as DSM), twisted nematic mode (hereinafter is referred as TN) and guest-host mode (hereinafter is referred as GH) are considered. Among these modes, TN liquid crystal has a characteristic of small driving power, high contrast and bright display.
Now description is made on the conventional color liquid crystal display apparatus configurated by a TN liquid crystal and color filter elements of red (hereinafter is referred as R), green (hereinafter is referred as G) and blue (hereinafter is referred as B).
Next, let us enter into the configuration of optical characteristic of TN liquid crystal which is an essential matter for the present invention is described.
FIG. 1 shows principle of display by transparent type TN liquid crystal display apparatus. A liquid crystal 1 of nematic type, a pair of paralelly disposed transparent substrates 2a and 2b and a liquid crystal 1 combined therebetween together constitute a liquid crystal cell, and a pair of polarizer 3a and 3b disposed with the polarizing axises in each other vertical relation are provided on the substrates 2a and 2b, respectively. A thick white arrow indicates direction of incident light and its polarization.
In the device of FIG. 1, in a 0 voltage impression state, the liquid crystal stops light as shown in FIG. 1(a). When a voltage above a threshold value (hereinafter is referred as V.sub.th) is applied, the liquid crystal is realigned in the direction of the electric field (provided) that the dielectric constant anisotropy of the liquid crystal is positive), and incident light from the upper substrate passes through the liquid crystal apparatus. Accordingly by application or removal of voltage across the electrodes, contrast of brightness and darkness is obtainable. The above-mentioned type the liquid crystal apparatus wherein a dark state is represented at non-impression of voltage across the electrodes on both substrates is defined as "normally black" type.
In actual apparatus, transmittance T at 0 volt impression is not 0. As a result of the optical rotatory dispersion, a linearly polarized light incident to the cell changes into elliptically polarized wave light and hence passes the cell. Transmittance of parallel light incident normally to the liquid crystal cell at the 0 volt impression state is given by the following equation (C. H. Gooch, H. A. Tarry: J.Phys.D:Appl.Phys.8,1575(1975)): EQU T=(1+u.sup.2).sup.31 1 sin .sup.2 [.theta.(1+u.sup.2).sup.1/2 ](1),
wherein EQU u=.pi.d.DELTA.n/.theta..lambda. (2),
wherein d is thickness of the liquid crystal layer, .DELTA.n is defraction anisotropy of the liquid crystal, .theta. is twist angle of the TN liquid crystal and .lambda. is wavelength of incident light.
Then, by providing .theta.=.pi./2, the value of u which satisfy T=0 of the equation (1) is given by: ##EQU1## wherein m is a positive integer.
In general, the defraction anisotropy .DELTA.n of the liquid crystal has a dependency on the wavelength of the incident light. FIG. 2 shows the wavelength-dependency of the defraction anisotropy .DELTA.n of a liquid crystal, for example, LIXON 9150 manufactured by Chisso Kabushiki Kaisha. FIG. 3 is a graph made by plotting for the example of d=5 .mu.m and d=8 .mu.m from the equation (1) by Gooch-Tarry of a spectral transmittance characteristic for 0 volt impression in a TN liquid crystal cell having the twist angle .theta. of 90.degree..
As shown in FIG. 3, the spectral transmittance characteristic shows about 10% transmittance at its peak in a visible range (400-700 nm) and can not completely stop the light. Accordingly, the cell of d=5 .mu.m shows reddish colors and the cell of d=8 .mu.m shows yellow colors. But the 5 .mu.m thick cell can stop the light around the wavelength of 570 nm and the 8 .mu.m thick cell can stop the light around 440 nm.
As shown in this actual example, when using the TN mode liquid crystal, there is problems of insufficient light stopping and resultant coloring in displaying. Apart from mono-color displaying wherein such insufficient light stopping and coloring is only little problem, these problems induce a great obstacle for full color displaying.
FIG. 4 shows a sectional view of a conventional color liquid crystal display apparatus. As shown in FIG. 4, the apparatus has a color filter having R, G and B color filter elements disposed in a matrices as shown in FIG. 5, and the apparatus has alignment films 6a and 6b formed on the inner wall of the substrate so as to control initial alignment of the liquid crystal at 0 volt impression state. Therefore by impressing a voltage above the threshold voltage V.sub.th across the transparent conductive film 5a and 5b, the molecular alignment of the liquid crystal is changed, so as to modulate light passing through the liquid crystal cell. Accordingly, by impressing appropriate voltages above the threshold value V.sub.th for respective electrodes responding to the R, G, B color filter elements of the picture elements, a full color displaying can be made by red, green and blue addition color mixing method. FIG. 6 shows one example of spectral transmittance characteristic of the color filters of R, G and B color used in this conventional apparatus. In the conventional exmaple, the problems to consider are leak and undesirable coloring of light at 0 voltage impression. Since a contrast ratio is defined as [light transmittance for bright state]/ [light transmittance for dark state], in the apparatus of the conventional configuration the existance of light leaking in the dark state, i.e., 0 volt impression state results in lowering of the contrast ratio.
In the conventional configuration where the thickness d of the liquid crystal layer is uniform for all parts of color picture elements for R, G and B, when for example d is 5 .mu.m, in the elements of the color filters G and R the lights are almost stopped at the 0 voltage impression, while the light cannot be stopped at the elements of the B filter parts as is obvious in FIG. 3 and FIG. 6. Accordingly the apparatus shows blue or violet color as a whole at the 0 voltage impression instead of a perfect black. This phenomenon is a grave obstacle in full color displaying by the color liquid crystal display apparatus.
As can be understood from the Gooch-Tarry's equation (1), as the thickness of the liquid crystal layer increases, (e.g. above about 10 .mu.m ) , the above-mentioned light leakage at 0 voltage impression decreases, and accordingly the coloring at the 0 voltage impression becomes small. However, when the thickness "d" increases, such problems are induced that elongation of response time of the liquid crystal for ON-OFF switching of the voltage impression, decrease of view angle and negative display voltage region as will be mentioned later which induces shift of color by variation of the view angle. Accordingly, such liquid crystal display apparatus becomes quite poor in color performance characteristics.
Since a color liquid crystal display apparatus using the TN mode liquid crystal is expected to have good response speed and wide view angle, the apparatus is liable to be designed with the relation between d and .DELTA.n to fulfil u.perspectiveto..sqroot.3 for green light, namely with as short thickness d as possible. However, when such a thin liquid crystal layer is used, there is a problem that light transmittance for having wavelength of the condition apart from u=.sqroot.3 becomes dominant, and then the display apparatus shows undesirable and unnatural colors.
As far as the liquid crystal layer of uniform thickness for different color parts is used, resolving of the above-mentioned various problems is impossible.